1. Field of the Invention
The present invention is directed at a method for removing sulfur compounds, such as sulfides and mercaptans from a hydrocarbon fluid. More specifically, the present invention is directed at a method for contacting the sulfide and mercaptan components with caustic to at least partially remove them from the hydrocarbon stream. The resulting sulfidic spent caustic effluent is carbonated, and the caustic is electrolytically recovered.
2. Description of the Prior Art
In petroleum and chemical processing operations, hydrocarbon feedstocks frequently contain sulfur compounds such as sulfides and mercaptans. These sulfur compounds often are corrosive, especially in aqueous environments. Moreover environmental regulations often limit the concentration of sulfur that can be discharged to the atmosphere. Therefore, it is desirable to reduce the concentration of sulfides and mercaptans to relatively low levels. One method which has been used to reduce the sulfur concentration in a hydrocarbon fluid has been to contact the fluid with an aqueous caustic solution. The resulting sulfidic spent caustic (SSC), containing high levels of sulfides and mercaptides, may be odorous and toxic. Accordingly, it may not be possible to discharge the SSC directly into typical plant wastewater treatment facilities. Several methods have been used to remove the sulfur compounds. U.S. Pat. No. 4,041,129 discloses a method of neutralizing the SSC with sulfuric acid at a pH such that the sulfides and mercaptides can be easily removed by stripping. The resulting aqueous sodium sulfate effluent is regenerated by passing this solution into an electrolytic cell to convert the aqueous sodium sulfate solution into sodium hydroxide and sulfuric acid. However, this process has several inherent disadvantages. The concentration of sulfuric acid used must be closely monitored. Use of an insufficient amount of sulfuric acid may result in incomplete stripping of the sulfides and mercaptides, while use of an excessive amount of sulfuric acid may cause an unacceptably high rate of corrosion of the process equipment. Moreover, a substantial portion of the process equipment may have to be constructed of stainless steel to reduce the rates of corrosion and the semi-permeable anionic membrane used in the electrolytic cell may have an unacceptably short life. Furthermore, the raw material requirements and power consumption may be high. U.S. Pat. Nos. 2,794,768 and 2,794,769 describe processes for removing H.sub.2 S and mercaptans from hydrocarbon feedstocks by contacting the feedstocks with an alkali hydroxide to form a spent sulfidic caustic. The SSC is regenerated by the addition of epoxides and the electrolytic conversion of the mercaptides to insoluble disulfides. However, these processes are relatively complex and require the use of epoxides. Moreover, the sulfidic compounds may foul the electrolytic membranes and decrease the electrical efficiency of the cell. U.S. Pat. No. 2,654,706 describes a process for removing sulfur compounds from gasoline by contacting the gasoline with sodium hydroxide. The sodium hydroxide then is passed through an electrolytic cell where the mercaptides present are converted to disulfides and the hydrogen sulfide is converted to insoluble sulfur. The sulfur compounds are then separated from the regenerated sodium hydroxide. This method uses relatively large quantities of electricity to electrolytically regenerate sodium hydroxide having sulfur compounds therein and does not produce high purity sodium hydroxide.
Yet another method to neutralize the SSC has been to use flue gas carbonation. Carbon dioxide from a flue gas source, such as the off-gas from a catalytic cracking unit used to process hydrocarbons, may be used to neutralize the SSC to enable the sulfides and mercaptides to be stripped. This carbonation has been commercially practiced on both a batch and a continuous basis. In the batch process, flue gas is sparged through the SSC to convert the sulfides and mercaptides to volatile sulfur compounds, the sparging continuing until the sulfides and mercaptides having been removed to a sufficiently low level. In the continuous process, the flue gas contacts and neutralizes the SSC in one tower. The solution subsequently is passed to a second tower where the sulfides and mercaptides are stripped off. In both carbonation systems the liquid effluent conventionally is sent to downstream wastewater treating facilities. However, at some locations, this may not be possible, as where the inorganic or organic constituents in this effluent cause operational upsets in the wastewater treatment facilities or where the dissolved salts in this effluent stream are unacceptably high.
The subject invention is directed at the carbonation of the SSC stream and the electrolytic regeneration of the resulting liquid effluent. Utilization of the subject invention results in a reliable process having relatively low raw material and power requirements which also has only a minimal effect on plant wastewater facilities. The process results in the formation of a relatively high purity alkali metal hydroxide which may be reused in the process or utilized in other operations which require a higher purity alkali metal hydroxide.